The advantages of oxy-fuel combustion systems are well recognized. For example, Gross, U.S. Pat. Nos. 6,436,337 and 6,596,220, provide that some of the advantages of oxy-fuel combustion systems are reduced environmental pollution (reduced NOx generation), high efficiency, high flame temperatures and smaller overall physical plant design. The Gross patents, which are commonly owned with the present application are incorporated herein by reference.
In order to extract the energy from the fuel, boilers typically provide some manner in which energy is input to a fluid (through combustion of the fuel) generally to change the state of the fluid. Energy is then extracted from the fluid typically in the form of mechanical movement (or kinetic energy). Most boilers use water as the working fluid to extract energy from the fuel. Water is passed through tubes that form one or more “walls” or bundles within the boiler.
Typically, boiler tube walls are designed to transfer energy (in the form of heat) through the tube wall into the water in several loops and passes of the walls. As the water passes through the tubes, the water is heated, under pressure and brought to a high level of energy (and phase change) through super-heat, re-heat and/or super critical stages. Other stages, such as an economizer unit may also be used through which water is passed in furnace wall sections prior to super-heat passes. The water is further heated by convective heat transfer from the heated gases flowing past the tube bundles (e.g., in the economizer).
Each of the stages or regions of the boiler is designed to operate based upon a certain type of heat transfer mechanism or phenomena. For example, the lower furnace walls are designed for radiant heat transfer whereas the upper bundles, super-heat, re-heat and economizer sections are designed to function on a convective heat transfer principle. It will be recognized by those skilled in the art that the heat transfer mechanisms are not exclusive of one another as water is heated in the boiler.
Although such boiler configurations continue to serve their applications and purposes well, they do not necessarily take full advantage of the high flame temperatures and low exhaust gas volumes of oxy-fuel combustion systems. Accordingly, there is a need for a boiler that uses an oxy-fuel combustion system to reduce environmental pollution. Desirably, such a boiler design provides high efficiency (vis-á-vis a high ratio of heat transferred to the working fluid to the heat available from the combustion products) and makes use of high flame temperatures. Most desirably, such a boiler configuration can provide a smaller overall physical plant design.